In general, a reluctance machine can be operated as an electric motor in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. the inductance of the exciting winding is maximized.
In one type of reluctance machine the energization of the phase windings occurs at a controlled frequency. These machines may be operated as a motor or a generator. They are generally referred to as synchronous reluctance motors. In a second type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor's position. This second type of reluctance machine may also be a motor or a generator and such machines are generally known as switched reluctance machines. The present invention is generally applicable to switched reluctance machines, including switched reluctance machines operating as motors or generators.
FIG. 1 shows the principal components of a switched reluctance drive system 10 for a switched reluctance machine operating as a motor. The input DC power supply 11 can be either a battery or rectified and filtered AC mains. The DC voltage provided by the power supply 11 is switched across the phase windings of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive 10. As such, a rotor position detector 15 is typically employed to supply signals corresponding to the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required. In other systems, the rotor position detector 15 can comprise a relative position encoder that provides a clock pulse (or similar signal) each time the rotor rotates through a preselected angle.
In systems where the rotor position detector 15 comprises a rotor position transducer, failure of the rotor position transducer circuitry to properly provide output signals representative of the angular position of the rotor can seriously degrade the performance or, in the worst case, render the motor inoperable. In some circumstances, a controller 14 attempting to control a machine based on faulty rotor position transducer outputs could potentially damage both the machine and the remainder of the control circuitry.
The importance of accurate signals from the rotor position detector 15 may be explained by reference to FIGS. 2 and 3. FIGS. 2 and 3 explain the switching of a reluctance machine operating as a motor.
FIG. 2 generally shows a rotor pole 20 approaching a stator pole 21 according to arrow 22. As illustrated in FIG. 2, a portion of a complete phase winding 23 is wound around the stator pole 21. As discussed above, when the portion of the phase winding 23 around stator pole 21 is energized, a force will be exerted on the rotor tending to pull rotor pole 20 into alignment with stator pole 21.
FIG. 3 generally shows the switching circuitry in power converter 13 that controls the energization of the portion of the phase winding 23 around stator pole 21. When power switching devices 31 and 32 are switched ON, phase winding 23 is coupled to the source of DC power and the phase winding is energized.
In general, the phase winding is energized to effect the rotation of the rotor as follows: At a first angular position of the rotor (called the turn-ON angle), the controller 14 provides switching signals to turn ON both switching devices 31 and 32. When the switching devices 31 and 32 are ON the phase winding is coupled to the DC bus which causes an increasing magnetic flux to be established in the motor. It is this magnetic flux pulling on the rotor poles that produces the motor torque. As the magnetic flux in the machine increases, electric current flows from the DC supply provided by the DC bus through the switches 31 and 32 and through the phase winding 23. In some controllers, current feedback is employed and the magnitude of the phase current is controlled by chopping the current by switching one or both of switching devices 31 and/or 32 on and off rapidly.
In many systems, the phase winding remains connected to the DC bus lines (or connected with chopping if chopping is employed) until the rotor rotates such that it reaches what is referred to as the rotor "freewheeling angle." When the rotor reaches an angular position corresponding to the freewheeling angle (position 24 in FIG. 2) one of the switches, for example 31, is turned OFF. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the return diodes (in this example 34). During the freewheeling period there is little voltage differential across the phase winding, and the flux remains substantially constant. The motor system remains in this freewheeling condition until the rotor rotates to an angular position known as the "turn-OFF" angle (represented by position 25 in FIG. 2). When the rotor reaches the turn-OFF angle, both switches 31 and 32 are turned-OFF and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux (and therefore the phase current) in the machine to decrease.
The energization of the phase windings in a switched reluctance motor depends heavily on accurately detecting the angular position of the rotor. If the rotor position detector fails and the controller continues to energize the phase windings, dangerously high currents could build up in the motor, potentially damaging the motor and the controller. Moreover, when a drive system fails, it is often necessary to test various control and motor components to find the failed elements. It would be beneficial to have an indicator that specifically indicates that the failure of the motor was the result of a rotor position detector failure so that unnecessary testing and debugging is not attempted. While some complicated rotor position detectors have some fault indicating circuits, such encoders are relatively expensive and require additional hardware for proper operation. Known position decoders do not provide a low cost, compact rotor position detector that provides an indication when the rotor position detector has failed.
In addition to problems with detecting sensor errors, known encoder systems for switched reluctance machines are often limited because of the costly electronics required to rapidly process digital signals provided by an incremental position encoder such that the phase energization occurs at the appropriate times. For example, in known systems, an incremental position encoder may be used that provides a relatively large number of digital clock pulses each complete revolution of the rotor. In systems that do not use costly electronic circuits or high speed microprocessors, it is often difficult and expensive to process the large number of digital pulses provided by the incremental encoder to properly synchronize the energization of the phase windings with the angular position of the rotor.
It is an object of the present invention to overcome the above described and other disadvantages of known position detectors and to provide a relatively inexpensive rotor position detector that provides an indication when a fault has occurred without the need for complex or expensive additional circuitry. Moreover, the present invention provides a rotor position encoder including an incremental and an absolute encoder and a method for efficiently controlling phase energization through the use of a repeating incremental position signal that comprises digital pulses of a number that is greater than the total number of changes of state that occur in the absolute encoder for each revolution. The use of this repeating signal from the incremental encoder allows for the construction of a low cost, efficient motor controller.